The invention relates to a component for membrane electrolysis cells, and is particularly directed to an insulating frame provided with a structured internal section allowing the penetration of a process electrolyte also in the regions in direct contact with the membrane. Under another aspect, the invention is directed to an electrolysis cell equipped with such micro-structured insulating frame.
Several types of electrolysis cells for the production of chlorine and hydrogen gas and/or caustic soda solution are known in the art. In particular, the most common cell designs in existing industrial applications are the filter-press type and the “single cell element” type, in which the elements are electrically connected in series.
The single cell element design, which is for instance disclosed in DE 102 49 508 A1 and DE 10 2004 028 761 A1, is comprised of anodic or cathodic semi-shells housing the respective anode and cathode. An ion-exchange membrane is positioned between the electrodes and kept in place by suitable flanges. As specified in DE 10 2004 028 761 A1, an insulating frame is arranged between the flange of the anodic semi-shell and the membrane, so that the membrane is clamped between the surfaces of the cathodic semi-shell and the insulating frame and held in position accordingly.
Since the membrane, which typically comprises a sulphonic layer and a carboxylic layer, is not tensioned during the cell assembly procedure but is simply placed horizontally on one of the semi-shells, the insulating frame also serves to prevent it from oscillating and coming in contact with the metallic surfaces of the anodic semi-shell during operation. In this regard, the transitional area between the anodic semi-shell and the flange is of special importance to prevent short-circuits and to protect the membrane from damages. For the above reasons, the insulating frame is oversized so that it protrudes by a few millimetres into the internal compartment and separates the membrane from the adjacent metallic surfaces of the semi-shell.
The detrimental effect of this safety measure is the deactivation of the membrane in the contact area. Since the pressure in the cathodic compartment is higher than that in the anodic compartment, the membrane is pressed towards the anodic compartment and/or against the protruding region of the frame, and thus it can be wetted only on the opposite side in the contact area.
On account of this blinding phenomenon on the anode side, the hygroscopic caustic solution present on the cathode side tends to dehydrate the membrane in this region, thus causing precipitation of salts in the carboxyl layer eventually leading to blistering, delamination of the two membrane layers and/or fissuration phenomena. These damages are sometimes visible, but they may also be detected by a high chloride concentration in the caustic product, owing to the migration of chloride ions to the cathodic compartment by diffusion through the damaged area. The efforts carried out so far to overcome this detrimental effect by improving the sizing or the positioning of the insulating frame were not satisfactory, so that either a higher chloride concentration is tolerated for long periods or the membrane has to be replaced more frequently.
It is one of the objects of the present invention to reduce damage to the peripheral region of the membrane by minimising the flux of chloride ions to the cathode side or by preventing it at all.
This and other objects which will be evident to those skilled in the art are achieved by the technical solution disclosed in the appended claims.